The field of the invention is collimators for use with detectors of radiation in medical imaging equipment, and particularly, collimators for gamma cameras.
Collimators are used to form images of a gamma ray emitting source on gamma ray detector elements mounted in a gamma camera. Collimators are positioned between the detectors and the source of the gamma radiation, and may comprise, for example, a slab of lead through which an array of holes is formed. In the alternative, a honeycomb structure formed from interlocked pieces of gamma ray attenuating material are also commonly used. Ideally, gamma rays emanating directly from the source and incident normal to the face of the detector array pass through the collimator, while all other gamma rays are absorbed by the attenuating material. Such gamma cameras are described, for example, in U.S. Pat. Nos. 3,890,506; 3,919,556; 3,191,557 and 4,582,994 which are assigned to the assignee of the present invention.
The performance of a collimator is measured by a number of factors including: the spatial resolution of the resulting image; the ability to block gamma rays at various energy levels; and the sensitivity, or geometric efficiency of the structure. These characteristics are determined by the attenuation coefficient of the collimator material, the size of the holes, the thickness of the walls, or "septa", separating the holes and the length of the holes.
In nuclear medicine, isotopes emitting gamma rays of differing energy are employed to study various organs of the human body. For higher energy gamma rays the geometry of the collimator is different than that of a collimator made from the same material for use at lower energy levels since more material (i.e. lead in the above example) is needed to block high energy gamma rays. As a result, in a typical installation several collimators are required to deal with the expected range of gamma ray energies produced by commonly used isotopes and to maintain the desired sensitivity and resolution.
Changing the collimator is a time consuming process. In addition to the blocking material, a surrounding steel or aluminum collar for mounting the collimator to the gamma camera adds to its weight and size. The resulting mass is typically 20 to 40 kg and requires about 30 minutes of machine down time to change and balance.